Effects of N1 and N5 alkyl substituents on the stability of 6-oxoverdazyl radicals

A series of 1,5-dialkyl-6-oxoverdazyl radicals with differing N1 and N5 substituents were synthesized as a means to observe the effects of different alkyl groups on the reactivity of verdazyl radicals towards disproportionation with a methyl group as the benchmark. Using previously described cycloaddition chemistry with verdazyl radical-derived azomethine imines via disproportionation, methyl acrylate was used to trap the disproportionation products in situ in order to observe the N1 versus N5 site-selectivity of this process. Surprisingly, in most cases the larger alkyl groups proved to be the more reactive as compared to methyl explained by the weakening of the C–H bond involved in a hydrogen atom abstraction as part of the disproportionation. These results contrast with the idea that greater steric bulk generally makes radicals less reactive.